Block copolymers (BCPs) are characterized by their ability to spontaneously self-assemble into dense periodic nanostructures having domains with small length scales. In the self-assembly process, factors which govern the size of the domains are the degree of polymerization (N) and the Flory-Huggins interaction parameter (χ) which is a measure of thermodynamic interactions between the polymer blocks. If χN is below a critical value, then the BCP will be disordered. Thus, for a given χ, the degree of polymerization can only be decreased a certain amount until the BCP no longer self-assembles. However, the resulting domain size correlates quite closely with N, thereby rendering this approach less useful for generating small domain sizes for nanolithography.
χBCP can be approximated as the degree of immiscibility between the blocks of the BCP. The most straightforward way to increase χBCP, therefore, is to increase the difference in polarity between the blocks. For a rough approximation of polarity, solubility parameters can be useful as they have been tabulated for a broad range of homopolymers.
Poly(4-hydroxystyrene) [P(4-HS)] has a high solubility parameter (24.55 (J/cm3)1/2) indicating a high degree of polarity and hydrophilicity. Due to the acidic and reactive nature of the phenol group, 4-hydroxystyrene (HS) has been protected before polymerization using a variety of groups. 4-acetoxystyrene, for example, has been employed in living free radical polymerizations and is deprotected by alkaline hydrolysis. (See, Kanagasabapathy, S.; Sudalai, A.; Benicewicz, B. C., Macromol. Rapid Commun. 2001, 22, 1076-1080 and Barclay, G. G.; Hawker, C. J.; Ito, H.; Orellana, A.; Malenfant, P. R. L.; Sinta, R. F., Macromolecules 1998, 31, 1024-1031.) However, anionic polymerization requires more stringent protection, hence monomers such as 4-tert-butoxystyrene and 4-tert-butyldimethylsilyloxystyrene have been used and deprotected under reflux with strong acid or reaction with fluoride anion. (See, Se, K.; Miyawaki, K.; Hirahara, K.; Takano, A.; Fujimoto, T., J. Polym. Sci., Part A: Polym. Chem 1998, 36, 3021-3034 and Ito, H.; Knebelkamp, A.; Lundmark, S. B.; Nguyen, C. V.; Hinsberg, W. D., J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 2415-2427.) Unfortunately, many polymer blocks that might be polymerized with PHS to provide useful BCPs are degraded by strong acids and attacked by fluoride anions.